Many proposals have been made to reinforce or otherwise improve the properties of concrete by applying various types of fibers, including asbestos, glass, steel, as well as polymer fibers to aqueous based concrete mixes prior to the curing of the concrete. The types of polymer fibers in use or proposed for use include those composed of polyolefins, especially polypropylene, polyester, polyamide, polyacrylic and polyvinyl alcohol.
Relatively large diameter fibers, for example, in excess of 40 to 60 microns in diameter, can be added to wet concrete, dispersed in the concrete by mixing, followed by pouring and curing of the concrete. Large diameter fibers serve to reinforce the concrete after it has been cured, by providing additional tensile strength and minimizing impact damage and crack propagation.
Small diameter fibers, typically less than 30 to 40 microns in diameter, and having a relatively high surface area, are commonly added to concrete mixes in order to reduce the development of small cracks in the concrete during the curing period. The problem of crack development is known to occur as a result of uneven curing of the concrete.
Polypropylene fibers are produced by a well known melt spinning process, in which molten polymer is pumped through a die having a large number of small openings to produce a tow of continuous filaments. The filaments are cooled and drawn or elongated to increase tensile strength. A size or finish is usually applied to the filaments, followed by drying and tempering if required and chopping into the desired length to provide bundles of fibers. The use of polypropylene fibers is desirable for several reasons, including low raw material cost, excellent physical properties, and the nonreactive properties of the polymer in the alkaline concrete mix.
Several problems are associated with the handling of polypropylene fibers and the uniform dispersion of the fibers into the concrete mix. The fibers are hydrophobic in the sense that they do not absorb water and have a surface which cannot be wetted. As a result, difficulties can arise in obtaining a uniform dispersion of the fibers in the cement mixture using conventional mixing equipment. Another problem which may be encountered, especially with the use of fine fibers, is that the fibers become entangled either during routine handling, packaging and transport, or after they are added to the concrete mix. As a result, the fibers do not disperse evenly and may form clots.
U.S. Pat. No. 5,399,195 discloses the addition of small amounts of fine (less than 30 microns) polymer fibers to concrete. During production, the filaments are treated with a topical wetting agent. After the filaments are chopped into fibers, the wetting agent holds or binds the fibers together in the form of bundles. The bundles remain relatively stable during handling, and when the fibers are added to the concrete mix, the wetting agent promotes dispersion of the fibers. Wetting agents are also commonly applied to polypropylene fibers used to make nonwoven fabrics.
The topically applied wetting agents described above are usually applied as an aqueous solution followed by drying, in order to provide a coating of the wetting agent on the fiber. The coatings are thus water soluble and are easily washed away, especially under high shear conditions of concrete mixing, causing the fibers to return to a hydrophobic state. Also, some types of wetting agents may cause undesirable foaming in the concrete mixture.
Despite recent advances in the art of concrete conditioning and reinforcement, a need exists for additional improvements and refinements, especially in the case of polypropylene fibers, which will provide a more reliable and more controllable technique for successfully providing fiber that will be uniformly dispersed in concrete under a wide variety of conditions.